1. Field of the Invention
The present invention relates to high frequency transistors and in particular relates to microwave field effect transistors (FETs) that incorporate nitride-based active layers.
2. Description of the Related Art
The present invention relates to transistors formed of nitride semiconductor materials that can make them suitable for high power, high temperature, and/or high frequency applications. Materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices for lower power and (in the case of Si) lower frequency applications. These more common semiconductor materials may not be well suited for higher power and/or high frequency applications, however, because of their relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 for GaAs at room temperature) and/or relatively small breakdown voltages.
GaAs based HEMTs have become the standard for signal amplification in civil and military radar, handset cellular, and satellite communications. GaAs has a higher electron mobility (approximately 6000 cm2/V-s) and a lower source resistance than Si, which may allow GaAs based devices to function at higher frequencies. However, GaAs has a relatively small bandgap (1.42 eV at room temperature) and relatively small breakdown voltage, which may prevent GaAs based HEMTs from providing high power at high frequencies.
In light of the difficulties presented by Si and GaAs, interest in high power, high temperature and/or high frequency applications and devices has turned to wide bandgap semiconductor materials such as silicon carbide (2.996 eV for alpha SiC at room temperature) and the Group III nitrides (e.g., 3.36 eV for GaN at room temperature). These materials typically have higher electric field breakdown strengths and higher electron saturation velocities as compared to gallium arsenide and silicon.
A device of particular interest for high power and/or high frequency applications is the high electron mobility transistor (HEMT), which is also known as a modulation doped field effect transistor (MODFET) or a Heterostructure Field Effect Transistor (HFET). These devices may offer operational advantages under a number of circumstances. They are typically characterized by the presence of a two-dimensional electron gas (2DEG) formed at the heterojunction of two semiconductor materials with different bandgap energies, where the smaller bandgap material has a higher electron affinity compared to the larger bandgap material. The 2DEG, which forms due to the presence of an accumulation layer in the smaller bandgap material, can contain a very high sheet electron concentration in excess of, for example, 1013 carriers/cm2 even though the material is nominally undoped. Additionally, electrons that originate in the wider-bandgap semiconductor transfer to the 2DEG, allowing a high electron mobility due to reduced ionized impurity scattering.
This combination of high carrier concentration and high carrier mobility can give the HEMT a very large transconductance and may provide a performance advantage over metal-semiconductor field effect transistors (MESFETs) for high-frequency applications, although MESFETs continue to be suitable for certain applications based on factors such as cost and reliability.
High electron mobility transistors fabricated in the gallium nitride (GaN) material system have the potential to generate large amounts of RF power because of the combination of material characteristics that includes the aforementioned high breakdown fields, their wide bandgaps, large conduction band offset, and/or high saturated electron drift velocity. In addition, polarization of GaN-based materials contributes to the accumulation of carriers in the 2DEG region.
GaN-based HEMTs have already been demonstrated. U.S. Pat. No. 6,316,793, to Sheppard et al., which is commonly assigned and is incorporated herein by reference, describes a HEMT device having a semi-insulating silicon carbide substrate, an aluminum nitride buffer layer on the substrate, an insulating gallium nitride layer on the buffer layer, an aluminum gallium nitride barrier layer on the gallium nitride layer, and a passivation layer on the aluminum gallium nitride active structure.
Improvements in the manufacturing of GaN semiconductor materials have focused interest on the development of GaN HEMTs for high frequency, high temperature and high power applications. GaN-based materials have large bandgaps, and high peak and saturation electron velocity values [B. Belmont, K. Kim and M. Shur, J. Appl. Phys. 74, 1818 (1993)]. GaN HEMTs can also have 2DEG sheet densities in excess of 1013/cm2 and relatively high electron mobility (up to 2000 cm2/V-s) [R. Gaska, J. W. Yang, A. Osinsky, Q. Chen, M. A. Khan, A. O. Orlov, G. L. Snider and M. S. Shur, Appl. Phys. Lett., 72, 707 (1998)]. These characteristics may allow GaN HEMTs to provide high power at higher frequencies.
A conventional GaN HEMT structure 110 is illustrated in FIG. 14. A channel layer 114 is formed on buffer layer 113 on a substrate 112. A barrier layer 116 is formed on the channel layer 114. A source electrode 118 and a drain electrode 120 form ohmic contacts through the surface of the barrier layer 116 to the electron layer that is present at the top of the channel layer 114. A gate electrode 122 forms a non-ohmic contact to the surface of the barrier layer 116.
Typically, the channel layer 114 includes GaN while barrier layer 116 includes AlGaN. Because of the presence of aluminum in the crystal lattice, AlGaN has a wider bandgap than GaN. Thus, the interface between a GaN channel layer 114 and an AlGaN barrier layer 116 forms a heterostructure or heterojunction where energy bands are deformed due to, for example, Fermi level alignment and polarization in the material.
FIG. 15 is an exemplary band diagram showing the energy levels in the device along a portion of section I-I′ of FIG. 14. As illustrated in FIG. 14, because the barrier layer 116 has a lower electron affinity (X) than the channel layer 114, when the Fermi levels in the materials align due to charge transfer, the energy bands of the channel layer 114 are shifted upwards, while those of the barrier layer are shifted downwards. As shown in FIG. 15, using properly designed materials, the conduction band Ec dips below the Fermi level (Ef) in the area of the channel layer 114 that is immediately adjacent to barrier layer 116, forming a narrow accumulation region. Consequently, a two dimensional electron gas (2DEG) sheet charge region 115 is induced in the accumulation region at the heterojunction between the channel layer 114 and the barrier layer 116. The barrier layer 116 is made sufficiently thin so as to be depleted of mobile carriers by the junction formed with the gate 122 and the resulting shape of the conduction band.
In addition, in a nitride-based device, the conduction and valence bands in the barrier layer 116 are further distorted due to polarization effects. This very important property of the heterostructures in the III-Nitride system may be essential for the high performance of the GaN HEMT. In addition to the accumulation of electrons due to the bandgap differential and band offset between the barrier and channel layers, the total number of free electrons is enhanced greatly by pseudomorphic strain in the barrier layer relative to the channel. Due to localized piezoelectric effects, the strain causes an enhanced electric field and a higher electron concentration than would, typically, be possible were the strain not present.
Electrons in the 2DEG sheet charge region 115 demonstrate high carrier mobility. Moreover, because the sheet charge region is extremely thin, the carriers are subject to reduced impurity scattering that may improve the device's noise characteristics.
The source to drain conductivity of this device structure is modulated by applying a voltage to the gate electrode 122. When a reverse voltage is applied, the conduction band beneath the gate is elevated, with the result that the conduction band Ec in the vicinity of the sheet charge region 115 becomes elevated above the Fermi level, and a portion of the sheet charge region 115 is depleted of carriers, thereby preventing or reducing the flow of current from source 118 to drain 120.
By forming the barrier layer 116 from AlN, certain advantages can be achieved. The 2.4% lattice mismatch between AlN (AlyGa1-yN for y=1) and GaN results in an increased and even maximum possible piezoelectric charge at the interface between the two layers. Using an AlN barrier layer also reduces the piezoelectric scattering between the layers that can limit the 2DEG mobility.
However, the high lattice mismatch between AlN and GaN dictates that the thickness of the AlN layer should be less than 50 Å. If the layer is thicker, the device can experience problems with its ohmic contacts, the material quality in the layer begins to degrade, the device's reliability decreases, and the material is more difficult to grow. However, a HEMT with a 50 Å or less AlN layer may be susceptible to high gate leakage.
Although GaN-based HEMTs have demonstrated exceptional power densities, a number of technical challenges still remain to be overcome before the devices can achieve commercial success. For example, one problem that may limit the performance and lifetime of certain GaN-based HEMTs is free carrier trapping, which may occur when carriers migrate away from the 2DEG region and become trapped in a surface dielectric region or in a buffer region beneath the channel. Such trapping may result in degradation in performance and/or reliability of a device.
Some attempts have been made to improve confinement of carriers within a HEMT channel by providing a second heterojunction below the channel—a so-called Double Heterostructure HEMT or DH-HEMT. However, in general, the amount of confinement due to the heterobarrier (which is a function of the difference in electron affinity between a wide-bandgap layer and the narrower-bandgap channel) may not be sufficiently large to result in effective confinement. Moreover, in a highly polarized material such as c-plane GaN, the polarization charges present in the material may reduce the confinement effect of the heterobarrier. Thus, in nitride-based transistor devices, the mere presence of a heterojunction alone below the channel may not be sufficient to effectively prevent carriers from migrating away from the 2DEG region into the buffer region where they can become trapped. Moreover, the structure of a DH-HEMT provides no additional barrier against surface trapping effects.
Another problem associated with the transit of carriers away from the channel region is linearity. When carriers are not confined to the channel, the ability to control their action via the applied gate voltage may be reduced, resulting in undesirable nonlinear transconductance characteristics.
The problems associated with free carrier trapping may also affect the performance of other types of nitride field effect transistors, such as GaN-based MESFETs.